Tolbutamide Stimulates Fructose-2, 6-Bisphosphate Formation in Perfused Rat Liver

Effect of tolbutamide on liver fructose-2,6-bisphosphate (F-2,6-P₂) was examined in isolated perfused rat liver in situ with a flow-through method. Tolbutamide (1 mM) gradually increased liver F-2,6-P₂ level from 7.4 ±1.6 to 21.2±1.6 pmol/mg wet wt for 20 min perfusion. The increase of liver F-2,6-P₂ induced by tolbutamide was dose dependent and was significantly observed at 10 min perfusion. The maximum plateau level of F-2,6-P₂ induced by 16.7 mM glucose was further increased with 1 mM tolbutamide. Glucagon (10⁻¹¹ M) decreased the elevated level induced by 16.7 mM glucose,but this effect was completely inhibited with 2 mM tolbutamide. Cyclic AMP level of the liver throughout the perfusion with tolbutamide did not change. Carboxytolbutamide or gliclazide perfusion did not change significantly the liver F-2,6-P₂ level; however, the results suggest that tolbutamide may increase the liver F-2,6-P₂ level by affecting the phosphorylation state of fructose-6-phosphate, 2-kinase/fructose-2,6-bisphosphatase through cyclic AMP—dependent protein kinase, resulting in the stimulation of glycolysis and the inhibition of gluconeogenesis in the liver. Thus, the extrapancreatic action and the mechanism of action of different sulfonylureas may differ.